Efficiency Questions

The temperature-entropy diagram of a reversible engine cycle is given in the figure. Its efficiency is:

A carnot engine takes $3\times 10^6\text{cal}$ of heat from a reservoir at $627^{\circ }\text{C}$, and gives it to a sink at $27^{\circ }\text{C}$. The work done by the engine is:

Helium gas goes through a cycle $ABCDA$ (consisting of two isochoric and two isobaric lines) as shown in figure. The pressure and volume at $A$ are $P_0,V_0$ and at $C$ are $2P_0,2V_0$. Efficiency of this cycle is nearly: